It is generally known that in the critical-care diagnosis and treatment of extremely sick persons, cardiac output CO (i.e. blood flow) and circulatory fill volume of a patient's circulatory system are important characteristics for monitoring the patient's state of health.
According to the current state of the art the cardiac output CO can be determined by using a dilution measurement. A bolus of an indicator defined by a predetermined quantity m of the indicator is injected into the patient's vena cava superior, and the indicator concentration c response is measured at a downstream location of the patient's systemic circulation, e.g. at patient's femoral artery. Based on the indicator concentration c response measurement versus time t the dilution curve is generated by plotting the indicator concentration c response as a function of time t.
A schematic example of the dilution curve is illustrated in FIG. 2, wherein the abscissa (time axis) and the ordinate (indicator concentration axis) are of linear scale.
Using the dilution curve, the cardiac output CO is defined as
      CO    =          m              ∫                  c          ⁢                      ⅆ            t                                ,wherein m is the indicator amount, c is the indicator concentration, and CO is the cardiac output.
As known from prior art, the sum of circulating volumes ΣVi between the location of injection of the indicator quantity m and the location of measurement of the indicator concentration c response is a specific parameter for estimating circulatory filling. Referring to FIG. 1, the sum of circulation volumes ΣVi encompasses the right-atrial end-diastolic volume V2, the right-ventricular end-diastolic volume V4, the blood volume of the lungs V1, left-atrial end-diastolic volume V3, the left-ventricular end-diastolic volume V5. The largest volume of the group of the circulation volumes V1, . . . ,5 is the blood volume of the lungs V1.
The sum of circulating volumes ΣVi can be derived from the dilution curve by calculating
                                          ∑                          V              i                                =                      CO            ·            MTT                          ,                                          MTT          =                                    ∫                                                c                  ·                  t                                ⁢                                  ⅆ                  t                                                                    ∫                              c                ⁢                                  ⅆ                  t                                                                    ,            wherein ΣVi is the sum of circulating volumes and MTT is the mean transit time defined as being the centre of mass of the dilution curve area.
It is generally known that the circulation volumes V2, . . . ,5 related to the heart indicate the circulatory fill status of a person. The sum of relevant circulation volumes ΣV2, . . . ,5 can be derived fromΣV2, . . . ,5=ΣVi−V1.
It is known from Newman et al (Circulation 1951 November; 4(5):735) that the largest volume V1 can be calculated by approaching a mono exponential function to the down slope part of the concentration curve. The time constant of this function is called down slope time DST. I.e., the down slope time DST is the time the indicator concentration takes to drop by the factor of e−1.
Therefore, the largest volume V1 can be calculated by
                                          V            1                    =                      CO            ·            DST                          ,                                          c          =                      const            ·                          ⅇ                                                -                  t                                DST                                                    ,            wherein CO is the cardiac output and DST is the down slope time.
It is common to use cooling energy as indicator for generating the dilution curve by injecting a cold liquid central venous and measuring the resulting temperature change in the aorta. In this case, dilution is called transpulmonary thermo-dilution.
In the case of transpulmonary thermo-dilution, ΣVi is called intra thoracic thermo volume ITTV, and the largest volume in the circulation V1 is called pulmonary thermo volume PTV.
In this respect, above mentioned equations can be read asITTV=CO·MTT,andPTV=CO·DST.
From U.S. Pat. No. 5,526,817 it is known that the global end-diastolic volume GEDV reflects the sum of the ventricle volumes, i.e. the sum of the smaller mixing volumes, without the lung volume, i.e. the largest mixing volume. These volumes essentially correspond to the end-diastolic cardiac volumes. The global end-diastolic volume GEDV can be determined from the thermo-dilution curve, i.e. from e.g. the difference between the intra thoracic thermo volume ITTV and the pulmonary thermo volume PTV, which could be derived from the difference between the mean transit time MTT and the down slope time DST, multiplied by the cardiac output CO, i.e.GEDV=ITTV−PTV=CO·(MTT−DST).
As mentioned above, when calculating the values of the cardiac output CO and the mean transit time MTT by making use of the thermo-dilution curve, the indicator concentration c is integrated over the time t. In order to get accurate results, the measurements of the indicator concentration over the time t are required not to be affected by interfering effects.
However, the decay part of thermo-dilution curve is superposed by blood recirculation flow. Therefore, on the one hand, it is useful to abort the measurement of the indicator concentration c before this recirculation flow occurs at the measurement location.
Further, the thermo-dilution curve converges to the mono exponential function as above mentioned just after a quite long period of time. Therefore, on the other hand, it is useful to calculate the down slope time DST at high time-coordinates.
Therefore, in order to overcome these disadvantages, usually, when performing the measurement of the indicator concentration c, the thermo-dilution curve is recorded until a level of 40% of the maximum dilution value is reached. Further, the down slope time DST is then estimated from the section of the dilution curve usually comprising 60% to 40% of the maximum dilution value by extrapolating the remaining decay curve (indicated in FIG. 2 as dotted line).
However, this extrapolation is erroneous. When having unfavorable circumstances, this down slope time DST value estimated by extrapolation of the dilution curve could differ up to 30% from the corresponding proper value at high time-coordinates.
Further, since the mean transit time MTT is defined as including a multiplication with the time t, errors in extrapolating the dilution curve are affecting the accuracy of the value of the mean transit time MTT.
Consequently, inaccurate values of the down slope time DST and the mean transit time MTT reduce the accurateness of the global end-diastolic volume GEDV calculated from these values.
Furthermore, referring to FIG. 2, the dilution curve is superposed by a baseline temperature drift. The dilution curve is asymptotically approaching the level of the baseline temperature extrapolation. In order to extrapolate the decay of the dilution curve properly, the baseline temperature drift has to be taken into account. The baseline temperature drift has to be estimated before the injection of the indicator.
The extrapolation of the baseline temperature drift is indicated in FIG. 2 as dotted line. With increasing time-coordinate the extrapolation of the baseline temperature drift becomes inaccurate thereby affecting the accuracy of the calculated values of the down slope time DST and the mean transit time MTT.